Electrical capacitor having electrically-conductive, impervious connector

ABSTRACT

An inter-cell, electron-conducting, ion-insulating connector for use in an electrical capacitor comprising a substrate of metal sheet or graphite which is laminated on at least one side with an elastomer containing electrically conductive carbon black in sufficient amounts to render the elastomer conducting.

United States Patent Isley 154] ELECTRICAL CAPACITOR HAVINGELECTRICALLY-CONDUCTIVE, IMPERVIOUS CONNECTOR [72] Inventor:

[73] Assignee:

[52] U.S. Cl ..317/230, 136/6 [51] Int.Cl. ..H01g9/00 [58] Field ofSearch ..317/230, 231, 232, 233; 136/6 [1 1 3,656,027 1 Apr. 11,1972

[56] References Cited UNITED STATES PATENTS 2,800,616 7/1957 Becker..317/230 3,105,178 9/1963 Meyers ..317/230 X 3,288,641 1 H1966Rightmire 136/6 3,536,963 10/1970 Boos ..317/230 Primary Examiner-JamesD. Kallam Attorney-John F. Jones and Sherman J. Kemmer [57] ABSTRACT Aninter-cell, electron-conducting, ion-insulating connector for use in anelectrical capacitor comprising a substrate of metal sheet or graphitewhich is laminated on at least one side with an elastomer containingelectrically conductive carbon black in sufficient amounts to render theelastomer conductmg,

10 Claims, 2 Drawing Figures CONTACT CEMENT Patented April 11, 1972 R OT C E N N O C S U m V R E D\ M PREPRESSED CARBON ELECTRODE POROUSSEPARATOR PREPRESSED CARBON ELECTRODE IMPERVIOUS CONNECTOR FIG.

CONTACT FIG. 2

INVENTOR. RALPH E. ISLEY fer/m 4. am

ATTORNEY ELECTRICAL CAPACITOR HAVING ELECTRICALLY- CONDUCTIVE,IMPERVIOUS CONNECTOR This invention relates to an improved electricalcapacitor and more particularly to an electrically conductive,impervious connector for use in an electrical capacitor having excellentstability at elevated temperatures.

The connector of this invention is characterized as anelectron-conducting and an interc'ell ionic-insulating member that ischemically inert to the corrosive electrolyte of the cell at elevatedtemperatures.

One of the most difficult problems to solve in achieving a pasteelectrode capacitor with temperature stability is finding a connectorthat is chemically inert to the corrosive electrolyte at elevatedtemperatures and at the same time contributes little to the overallequivalent series resistance of the system. Most metals that have lowcorrosion rates in electrolytes do so by forming a protective film onthe'surface of the metal. This film then insulates the electrode fromthe metal connector thereby resulting in a cell with high equivalentseries resistance. An obvious means for overcoming this problem ofprotective film formation on the surface of the metal is to plate themetal or metal alloy substrate with a layer of an inert noble metal suchas gold. However, in most cases this method is impractical from thestandpoint of cost.

The electrically conductive connector of this invention which meets therequirements for a capacitor operable in the range of -40 to 175 C.comprises a substrate of metal sheet, metal foil or graphite sheethaving laminated thereto on at least one side an electrically conductivepolymer. Preferably the polymer is one that can be blended with asufficient amount of electrically conductive carbon black to give thedesired level of conductivity. Because high concentrations of carbonblack are required for this purpose, elastomers are generally moresatisfactory than high modulus resins. The preferred elastomers arethose in which sufiicient amounts of carbon black can be added toincrease the conductivity or conversely to lower the resistance of theelastomer to within the range of lohm -in /mil, or so that theequivalent series resistance of the overall cell does not exceed 30milliohms.

The elastomers suitable for lamination for the connector of thisinvention are the natural rubbers and the synthetic rubbers obtainedfrom ethylene, propylene, isobutylene, bu.- tadiene, isoprene,chloroprene, copolymers of styrene-butadiene, copolymers of isobutylenewith various conjugated dienes wherein the amount of the diene does notexceed about 5 percent, as for example the butyl rubbers,chlorosulfonated polyethylene, vinylidene fluoride polymers,ethylenepropylene terpolymers, nitrile rubbers, and Thiokol rubbers.Preferred are the natural rubbers and the synthetic rubbers preparedfrom copolymers of butadiene with styrene or acrylonitrile, and mostpreferred are the butyl rubbers because of their ability to retain largeamounts of conductive carbon black and at the same time maintain theirother desirable properties.

The carbon blacks suitable for the purpose of this invention arepreferably finely divided, electrically conductive furnace blacks thatare produced by the thermal decomposition or incomplete combustion ofpetroleum oil or natural gas. Channel blacks or blacks made fine bygrinding, although operable in this invention, are less satisfactory forthis purpose. The carbon blacks are advantageously added to the rubberon a weight basis in concentrations ranging from 50 to 200 parts ofcarbon black per hundred parts of the rubber and preferably from 75 to175 parts of the carbon black per hundred parts of rubber.

The substrate of the connector may consist of any one of a number ofmetals or metal alloys in the form of a sheet or foil. The preferredmetals are those that contribute a minimum of resistance to theequivalent series resistance of the cell. Examples of suitable metalsinclude copper, and copper alloys such as brass, steel, niobium, tin,tin plate on steel, molybdenum, tantalum and zirconium. Especiallypreferred are nickel and nickel-containing alloys such as the Hastelloyalloys which are alloys consisting essentially of nickel, iron, andmolybdenum; the Monel metals which are alloys of predominantly nickeland copper; etc. Nickel and nickel alloys are preferred because of thelow interfacial resistances produced between the elastomer laminate andthe nickel substrate. Not to be excluded from substrate materials ofthis invention is graphite sheet, also found to be satisfactory for thispurpose.

In another aspect of this invention the substrate may comprise a loosenet of non-conducting woven fibers instead of metal. In such anembodiment the elastomer is superposed on a mat or a loose net of fiberscomposed of such materials as glass, asbestos, carbon, cellulose, etc.,for the purpose of preventing the elastomer from stretching. Metallicscreen is also suitable as a substrate material.

The rubber-carbon black blends may be applied to the sub strate by anyone of several methods known to those skilled in the art. For example,they may be applied by calendering, silk screening, spray coating, brushcoating, etc. For most applications the carbon saturated elastomer ismixed with a suitable hydrocarbon or substituted hydrocarbon solvent toobtain the desirable consistency for the particular method ofapplication employed. Solvent systems particularly satisfactory for thecarbon-saturated butyl rubbers are toluene and mixtures of toluene andthe xylenes.

The elastomer coating should be applied to the substrate in sufficientthickness to provide for adequate corrosion protection against theelectrolyte and at the same time not too thick a coating should beapplied so as to increase the overall electrical resistance of the cellbeyond the desirable limits. Generally, a thickness of at least 2 milsis found to be essential to provide the corrosion protection required,and the coating should not exceed a thickness of about 10 mils so as notto increase the resistance of the overall system beyond the 30 milliohmlimit. I

The laminate of the carbon-saturated elastomer may be applied directlyon the metal substrate or it may be desirable to improve itsadhesiveness by previously coating the substrate with a metal primer.Again the primer selected should be one that is electrically conductiveand one that contributes little electrical resistance to the system atthe temperature of operation of the cell. Examples of suitable primersinclude phenylethylethanol amine, trimethoxysilylpropethylene diamine,

tetraethyl ammonium toluene parasulfonate, gammaaminopropyltriethoxysilane, and in particular, polyethyleneimine.

It is advantageous to seal the elastomer laminate to the metal substrateby compression molding to improve corrosion protection and to lower theelectrical resistance of the coating. To further reduce the electricalresistance of the laminate on the metal substrate the elastomer coatingmay be cured by exposure to nuclear radiation, ultraviolet light,infrared radiation, steam, hot air and preferably vulcanization underpressure. In the vulcanizing procedure, the carbon-saturated elastomeris mixed with the usual vulcanizing agents such as accelerators,antioxidants, antiozonants, waxes, stabilizers etc. and is cured attemperatures of from room temperature to 360 F. and at pressures of from15 to 25,000 psi, for a period of time ranging from 5 minutes to 24hours, depending on the temperature of cure. For example, a laminate maybe cured at room temperature for a period of 24 hours.

The invention will be more readily understood from the followingdetailed description taken in conjunction with the drawings wherein:

FIG. 1 is an exploded view showing the parts of the capacitor.

FIG. 2 is a cross section of the assembled capacitor.

FIG. 1 represents one example of a single cell electrical capacitorconsisting of a pair of carbon paste electrodes 10, 11, a porousseparator 13, a pair of electron-conducting and ionic-insulating members14 comprising in combination a metal substrate 16 and laminated on atleast one side with a protective coating of an elastomer 15 having ahigh loading of conductive carbon black particles. Member 14 ischaracterized by its electrical-conducting properties and its chemicalinertness to the particular electrolyte employed at the potentialimpressed upon it. Its primary functions are as a current collector andan inter-cell ionic insulator.

An annular means or a gasket 12 is preferably cemented or in some manneraffixed to conducting member 14. Since paste electrodes and 11 are notrigid masses but are to some extent flexible, the principal function ofgasket 12 is to confine the electrodes 10 and 11 and prevent the mass ofthe electrode material from seeping out. The gasket is preferablyconstructed from an insulating material although it need not necessarilybe limited to this type of material. The gasket material should beflexible to accommodate expansion and contraction of the electrode.Other obvious ways of confining the electrode should be apparent tothose skilled in the art.

Separator 13 is generally made of a highly porous material whichfunctions as an electronic insulator between the electrodes yet allowsfree and unobstructed movement to the ions in the electrolyte. The poresof the separator 13 must be small enough to prevent contact between theopposing electrodes, since such a condition would result in a shortcircuit and consequent rapid depletion of the charges accumulated on theelectrodes. The separator can also be a non-porous ion-conducting film,such as an ion-exchange membrane. Any conventional battery separatorshould be suitable, and materials such as porous polyvinyl chloride,glass fiber filter paper, (Watman G.F.A.), cellulose acetate, mixedesters of cellulose, and fiber glass cloth have been found to be useful.Prior to its use it is advantageous to saturate the separator withelectrolyte. This can be accomplished by soaking the separator in theelectrolyte for a period of time of up to about 15 minutes.

The carbon electrodes 10 and 11 consist of activated carbon particles inadmixture with the electrolyte. Activation of carbon is the process bywhich adsorption properties and surface area are imparted to a naturallyoccurring carbonaceous material. Because electrical energy storage of acapacitor is apparently based on surface area, an increase in energystorage can be expected from an increase in surface area, as byactivation.

Active carbon, which is utilized in the preparation of the carbon pasteelectrodes, has surface area in the range of 100-2,000 meters /g, andpreferably in the range of 500-],500 meters lg as measured by theBrunauer-Emmett- Teller method. The surface area is mainly internal andmay be generated by numerous activation methods.

The initial stage in the preparation of an active carbon iscarbonization or charring of the raw material, usually conducted in theabsence of air below 600 C. Just about any carbon-containing substancecan be charred. After the source material is charred, the second step isactivation. The method used most extensively to increase the activity ofcarbonized material is controlled oxidation of a charge by suitableoxidizing gases at elevated temperatures. Most of the present commercialprocesses involve steam or carbon dioxide activation between 800 C and1,000 C, or air oxidation between 300 C. and 600 C. Alternately, gasessuch as chlorine, sulfur dioxide and phosphorous may also be used. Otheractivation methods include activation with metallic chlorides andelectrochemical activation.

Paste electrode 11 may also comprise a paste formed from the electrolytein admixture with solid particles of boron carbide or a refractory hardmetal carbide or boride wherein the metal may comprise tantalum,niobium, zirconium, tungsten and titanium, as more fully disclosed incopending application Ser. No. 71,852, filed Sept. 14, 1970. Also, pasteelectrode 11 may comprise a mixture of the electrolyte and a metalpowder of copper, nickel, cadmium, zinc, iron, manganese, lead,magnesium, titanium, silver, cobalt, indium, selenium and tellurium, asdisclosed in copending application, Ser. No. 101,834 filed Dec. 28,1970.

The electrolyte may consist of a highly conductive medium such as anaqueous solution of an acid, base or salt. In applications whereinconductivity of an electrolyte determines its selectivity, 30 percentsulfuric acid is especially preferred. Non-aqueous electrolytes can alsobe used. Solutes such as metal salts of organic and inorganic acids,ammonium and quaternary ammonium salts etc., may be incorporated inorganic solvents as, for example, nitriles such'as acetonitrile,propionitrile; sulfoxides such as dimethyl-, diethyl-, ethyl methyl-,and benzylmethyl sulfoxide; amides such as dirnethyl formamide,pyrrolidones such as N-methylpyrrolidone; and carbonates such aspropylene carbonate. Raymond Jasinski, High Energy Batteries andpublications of the Proceedings of Nineteenth and Twentieth Annual PowerSources Conference disclose other candidate nonaqueous electrolytes.

A detailed description of the electrical capacitor having carbon pasteelectrodes is more fully disclosed in US. Pat. No. 3,536,963. In theassembly of the cell, the component parts are assembled in the ordershown in FIGS. 1 and 2, and the same reference numerals are employed todesignate the same component parts in both figures. The cell is thencompressed at a pressure sufficient to render the cell a coherentstructure. Pressures in the range of about 240 psi have been foundsufficient for this purpose.

A stacked arrangement of single cell capacitors wherein the substrate ofthe electrically conducting, intercell connector is laminated on bothsides with elastomer is also contemplated to be within the scope of thisinvention.

EXAMPLE I An isobutylene isoprene rubber compound formulation wasprepared by mixing the following components in the proportionsindicated:

1.25 pans by weight 1 Butyl 365 Enjay Chemical Co.

2 Vulcan XC-72 Cabot Corporation 3 Captax R. T. Vanderbilt Co. lnc.

4 Methyl Tuads R. T. Vanderbilt Co. lnc.

A sample of the above material after mixing was sheeted out to athickness of 10 mils (0.010 inch) on a differential roll mill. This wasthen vulcanized onto a primed metal surface consisting of a nickel foildisc 1.125 inches in diameter and five mils thick. The primer compriseda 0.5 weight percent solution of polyethyleneirnine (5 PEI-18 DowChemical Co.) dissolved in ethanol. This was painted onto the nickelfoil and air dried for 30 minutes at 75 F and then air oven dried for 30minutes at F. The primed surface was covered with the above 10 mil sheetof butyl rubber compound, placed between two chrome plated metal sheetsin a hydraulic press maintained at a temperature of 320 F., and thepress closed at a total force of 25,000 lbs. The assembly was vulcanizedunder pressure at 320 F. fro 20 minutes. After calendering andvulcanizing, the nickel foil was coated with a 3 mil thick layer ofbutyl rubber vulcanized to its surface. This rubber layer was highlyconductive electrically.

Two butyl rubber-nickelfoil connectors prepared as above wereincorporated in a capacitor that was constructed and assembled as shownin FIG. 1. The capacitor contained a pair of carbon paste electrodes,1.125 inches in diameter, prepared from activated carbon (NucharActivated Carbon C-l l5, obtained from West Virginia Pulp and PaperCompany), having a surface area of 700-950 m /g; an electrolyteconsisting of 30 percent aqueous sulfuric acid; an ionically conductingseparator, 1.125 inches in diameter, and 3 mils thick, prepared from ananisotropic membrane consisting of two separate plastic layers ofmicroporous plastic sheet of the same polymer composition, one layerconsisting of a microporous sponge with a porosity of 50-80 percent, andthe other a consolidated layer with a porosity of l-3 percent, themembrane having a resistivity in 40 percent KOH of 3.8 ohm-cm; and apair of gaskets constructed from a copolymer of vinylidene fluoride andhexafluoropropylene, (Viton) having a thickness of 0.0 l 5 inch, an ID.of seven-eighths inches and an CD. of 1.125 inches. The assembled cellwas placed in a clamp and a 1.25 inch (I.D.) retaining ring slipped overthe capacitor and cylinder assembly. The cell was compressed under apressure of 240 psi. The equivalent series resistance of the capacitorwas measured and found to be 28 milliohms.

EXAMPLE II A capacitor was constructed as in Example I with theexception that the metal substrate of the connector consisted of coppersheet instead of nickel foil and the butyl rubber elastomer wasvulcanized directly on the copper without the use of a primer. Theequivalent series resistance of the capacitor after performing for l,000 hours was 40 milliohms.

EXAMPLE III A capacitor was constructed as in Example I with theexception that the metal substrate of the electron-conducting connectorconsisted of tin plate on steel. The equivalent series resistance of thecapacitor was equivalent to 20 milliohms.

' EXAMPLE iv EXAMPLE V A capacitor was constructed as in Example I withthe exception that the elastomer laminate of the electron-conductingconnector consisted of high density polyethylene containing 95 parts byweight of electrically conducting furnace black per 100 parts by weightof elastomer. The capacitor had an equivalent series resistance of 35milliohms. Electron-conducting connectors prepared from copolymers ofbutadiene with styrene and with acrylonitrile gave similar results.

EXAMPLE VI A capacitor was constructed as in Example I with theexception that the electron-conducting connector was constructed by silkscreening onto a nickel foil substrate a fluid paste formed bydissolving a mixture of 30 grams of butyl rubber containing 125 parts byweight of electrically conductive furnace black per 100 parts by weightof rubber in 70 mls. of a 5050 solvent mixture of toluene and xylene.The paste was applied onto the foil to a thickness of 2 mils. Theconnector thus constructed had a resistance of 22 milliohms.

I claim:

1. An electron-conducting connector including a substrate composed of amaterial selected from the group consisting of a metal and graphite, andan ion-insulating elastomer having electrically conductive carbon blackblended therein for imparting electrical conductivity thereto, saidelastomer and substrate adhering together and forming an electronicallyconductive ion-insulating, impervious laminate.

2. The connector of claim I wherein the elastomer is selected from thegroup consisting of butyl rubber, copolymers of butadiene-acrylonitrile,and butadiene-styrene, and the natural rubbers.

3. The connector of claim 2 wherein the elastomer consistsof butylrubber.

4. The connector of claim 1 wherein the elastomer is blended with fromabout to 200 parts by weight of electrically conductive carbon black per100 parts by weight of the elastomer.

5. The connector of claim 1 wherein the elastomer consists of butylrubber blended with from 75 to 175 parts by weight of electricallyconductive carbon black per 100 parts by weight of the butyl rubber, andthe substrate consists of a metal selected from the group consisting ofnickel and a nickel-containing alloy.

6. The connector of claim 5 wherein the butyl rubber-carbon black blendis laminated to a nickel substrate having been previously coated with apolyethyleneimine primer.

7. The connector of claim 6 wherein the butyl rubber-carbon black blendlaminated to the nickel substrate is cured by vulcanizing.

8. An electrical capacitor comprising a pair of spaced active carbonelectrodes compressed from a viscous paste of carbon particles and anelectrolyte; a porous ionically conductive separator saturated withelectrolyte between said electrodes; and an electron-conducting,ion-insulating connector impervious to the electrolyte positionedadjacent to the outerside of each of said electrodes, saidelectron-conducting, ion-insulating connectors consisting essentially ofan elastomer containing a sufficient amount of electrically conductivecarbon black to impart electrical conductivity thereto, laminated to asubstrate composed of a material selected from the group consisting of ametal and graphite, said elastomer being laminated to at least one sideof the substrate immediately facing the electrode.

-9. The electrical capacitor of claim 8 wherein the electrolyte consistsof 30 percent sulfuric acid.

10. The electrical capacitor of claim 9 wherein the elastomer of theelectron-conducting, ion-insulating connector consists of butyl rubberblended with from about to 175 parts by weight of electricallyconductive carbon black per parts by weight of the butyl rubber, and thesubstrate consists of a metal selected from the group consisting ofnickel and a nickel-containing alloy.

2. The connector of claim 1 wherein the elastomer is selected from thegroup consisting of butyl rubber, copolymers of butadiene-acrylonitrile,and butadiene-styrene, and the natural rubbers.
 3. The connector ofclaim 2 wherein the elastomer consists of butyl rubber.
 4. The connectorof claim 1 wherein the elastomer is blended with from about 50 to 200parts by weight of electrically conductive carbon black per 100 parts byweight of the elastomer.
 5. The connector of claim 1 wherein theelastomer consists of butyl rubber blended with from 75 to 175 parts byweight of electrically conductive carbon black per 100 parts by weightof the butyl rubber, and the substrate consists of a metal selected fromthe group consisting of nickel and a nickel-containing alloy.
 6. Theconnector of claim 5 wherein the butyl rubber-carbon black blend islaminated to a nickel substrate having been previously coated with apolyethyleneimine primer.
 7. The connector of claim 6 wherein the butylrubber-carbon black blend laminated to the nickel substrate is cured byvulcanizing.
 8. An electrical capacitor comprising a pair of spacedactive carbon electrodes compressed from a viscous paste of carbonparticles and an electrolyte; a porous ionically conductive separatorsaturated with electrolyte between said electrodes; and anelectron-conducting, ion-insulating connector impervious to theelectrolyte positioned adjacent to the outerside of each of saidelectrodes, said electron-conducting, ion-insulating connectorsconsisting essentially of an elastomer containing a sufficient amount ofelectrically conductive carbon black to impart electrical conductivitythereto, laminated to a substrate composed of a material selected fromthe group consisting of a metal and graphite, said elastomer beinglaminated to at least one side of the substrate immediately facing theelectrode.
 9. The electrical capacitor of claim 8 wherein theelectrolyte consists of 30 percent sulfuric acid.
 10. The electricalcapacitor of claim 9 wherein the elastomer of the electron-conducting,ion-insulating connector consists of butyl rubber blended with fromabout 75 to 175 parts by weight of electrically conductive carbon blackper 100 parts by weight of the butyl rubber, and the substrate consistsof a metal selected from the group consisting of nickel and anickel-containing alloy.